F1 mice undergoing GVH-associated immunosuppression as a result of parental T cell inoculation were treated with cyclophosphamide (Cy) at the time of parental cell inoculation. Such treatment prevented the development of immunosuppression. Furthermore, mice injected 37 days earlier with parental spleen cells were "rescued" in that their immune response potential was restored by injection of Cy. Inbred strains of mice injected with spleen cells from allogeneic strains and Cy exhibited antigen-specific reduced response potential to the H-2 antigens expressed by the strain used for injection.